A heat-sensitive recording material using an electron donating dye precursor and an electron accepting compound is disclosed in Japanese Patent Publication Nos. 14039/70 (corresponding to U.S. Pat. No. 3,539,375) and 4160/68. This heat-sensitive recording material is required essentially to have such properties that (1) color forming density and color forming sensitivity should be sufficient, (2) fog, which is the color forming phenomenon occurring during the storage of unused heat-sensitive recording materials, should not occur and that (3) color fastness, after color is formed, should be sufficient. However at present, satisfactory heat-sensitive recording material meeting the above-described requirements has not yet been obtained.
Particularly with recent demand for much higher speed recording by a heat-sensitive recording system, research as to the above-described (1) has been extensively developed.
As one approach to satisfying the above requirement (1), it has been suggested that the melting point of an electron accepting compound itself is to be raised from 60.degree. C. to 100.degree. C. However, it is difficult to adjust the melting point of phenol compounds which are most widely used as an electron accepting compound and, further, those phenol compounds are costly and thus are not practical.
As another approach, it is disclosed in Japanese Patent Publication Nos. 17748/74 and 39567/76 that organic acids and phenol compounds are used in combination, or polyvalent metal salts of compounds having alcoholic hydroxy groups are used as an electron accepting compound. As still another approach, it is disclosed in Japanese Patent Publication No. 29945/76 that a copolymer of hydroxyethyl cellulose and maleic anhydride is used as an electron accepting compound.
It is also disclosed in Japanese Patent Publication No. 27599/76 and Japanese Patent Application (OPI) No. 19231/73 (the term "OPI" as used herein means an "unexamined published application") that waxes are added.
It is further disclosed that in order to increase color sensitivity, a nitrogen containing compound such as acetoamide, stearoamide and the like (Japanese Patent Application (OPI) No. 34842/74), acetoacetic anilide (Japanese Patent Application (OPI) No. 106746/77), N,N-diphenylamine and benzamide derivatives (Japanese Patent Application (OPI) No. 11036/78), alkylated biphenyl and biphenyl alkane (Japanese Patent Application (OPI) No. 39139/78) and p-hydroxybenzoate derivative (Japanese Patent Application (OPI) No. 144193/81) are incorporated into a heat-sensitive color forming layer.
It is disclosed that into a heat-sensitive color forming layer, fatty acid amide wax is added (Japanese Patent Application (OPI) No. 115554/74), di-.beta.-naphthyl-p-phenylene diamine is added (Japanese Patent Application (OPI) No. 149353/75), and carboxylic acid ester such as benzoin, dimethyl isophthalate or diphenyl phthalate is added (Japanese Patent Application (OPI) No. 5636/78).
It is known that naphthol derivatives such as .beta.-naphthol benzyl ether, .alpha.-naphthol benzyl ether and the like and naphthoic acid derivatives such as phenyl .alpha.-hydroxy-.beta.-naphthoate are used as an sensitizing agent to increase the color forming sensitivity.
Among the above-described approaches, a method of using .beta.-naphthol-benzyl ether derivatives as a sensitizing agent and a method of using p-hydroxybenzoate as an electron accepting compound are the most effective.
However, the heat-sensitive recording material prepared by using the above-described compounds has such defects that color formed images disappear with passage of time (density decay) and that white powders appear on the surface of images (breeding).